Method of manufacturing a laminated rotary anode for use in an x-ray tube

ABSTRACT

Method of manufacturing laminated rotary anodes for use in x-ray tubes in which in a first step a laminated disc-shaped member is jointed by a single blow to another disc-shaped member, the diameters of the members being increased and their thicknesses decreased. The rotary anode is made from the resulting assembly. The laminated disc-shaped assembly consists at the target area, for example of a tungsten-rhenium alloy and at the other surface of tungsten or an alloy between capable of being shaped than the first mentioned tungsten-rhenium alloy.

This is a continuation of Application Ser. No. 621,484 filed Oct. 10,1975--now abandoned; which in turn is a continuation of Application Ser.No. 418,581, filed Nov. 23, 1973--now abandoned.

The invention relates to a method of manufacturing laminated rotaryanodes for use in X-ray tubes which comprise target areas mainlyconsisting of tungsten and supports mainly consisting of molybdenum, andto rotary anodes made by this method.

Netherlands Patent Application No. 6,916,885, corresponding to U.S. Pat.No. 3,735,458, (British Patent Specification No. 1,308,679) described amethod of manufacturing a laminated rotary anode for use in an X-raytube which comprises a target area for the electrons which is madesubstantially of tungsten and a support which is made substantially ofmolybdenum. In this known method a disc-shaped piece consisting mainlyof tungsten is joined to a disc-shaped piece consisting mainly ofmolybdenum by a high-speed forming impact process in which the diametersof the two disc-shaped pieces are increased and their thicknesses aredecreased. The stresses of the resulting member are relieved byannealing and the member is then shaped into the desired anode.

The term "high-speed forming impact process" in this connection is to beunderstood to mean a forming process in which a workpiece is shaped bymeans of a small number of blows, preferably by a single blow of largeenergy content, in a machine provided with plane die blocks. Machinesfor carrying out such a method are known and commercially available.Very good results are obtainable by means of a machine in which the dieblocks are moved together at a high speed driven by a compressed gas(generally referred to as a pneumo-mechanical machine).

The part of the surface of a rotary anode for an X-ray tube on which theelectrons impinge, which is generally referred to as the focal spottrack, is subjected to greatly fluctuating thermal stresses. Thesethermal stresses give rise to roughening of the focal-spot track andhence to a reduction of the X-ray output. As is known, the addition ofcertain high-melting point metals to tungsten which increase itsductility, enables the time of operation in which a certain degree ofroughening is produced with the same load, to be prolonged. One of thebest known alloying metals used for this purpose is rhenium, but theliterature further mentions tantalum, osmium, iridium, platinum,ruthenium, rhodium and palladium in this respect.

In the abovedescribed method it is necessary for both disc-shaped piecesto have their diameters increased to substantially the same extent bythe forming process. In the aforementioned patent specification it isstated that for this purpose the deformabilities of the disc-shapedpieces are matched by a suitable choice of their thicknesses, of thetemperature used and of the nature and the properties of the materialsthe pieces consist of.

It has been found in practice that sometimes the resulting assembliesare unsuitable for being made up into rotary anodes. This may be thecase, for example, if for satisfactory matching of the deformabilitiesthe thicknesses of the disc-shaped pieces have to be chosen so that thethicknesses of the layers obtained after forming are too small for theintended purpose. Also the layer thicknesses of the disc-shaped piecesdictated by the ultimate purpose may require the use of materials whichare less suitable, or are not the best choice, for the intended purpose.

Difficulties of this nature occur in particular when a disc-shaped piececonsists of a tungsten-rhenium alloy having a rhenium content in excessof about 2% by weight and the other piece consists of a molybdenum alloywhich in order to raise the recrystallisation temperature and toincrease the mechanical strength at elevated temperatures contains, forexample, small amounts of titanium and/or zirconium and optionallycarbon, as is the case with the molybdenum alloys available under thetradenames TZM and TZC. According to Braun in "Metall" 16, (1962), page992, TZM contains from 0.40% to 0.55% by weight of titanium, from 0.06%to 0.12% by weight of zirconium, the remainder being molybdenum, and TZCcontains about 1.25% by weight of titanium, from 0.15% to 0.25% byweight of zirconium and from 0.15% to 0.30% by weight of carbon, theremainder being molybdenum.

It is an object of the present invention to provide a method ofmanufacturing rotary anodes for use in X-ray tubes in which the abovedifficulties are avoided and by which rotary anodes for X-ray tubes areobtained in which the layers sufficiently adhere to one another and arethick enough for the intended use.

According to the invention this is attainable by means of a method whichis characterized in that a laminated disc-shaped member, which comprisesa layer consisting of a tungsten alloy suitable for use as a targetlayer for the electrons and a layer consisting of a metal having a loweryield point than the aforementioned tungsten alloy and consisting oftungsten or a tungsten alloy, is joined by the latter layer to thedisc-shaped piece mainly consisting of molybdenum by means of thehigh-rate impact shaping process.

By means of the method according to the invention a rotary anode for anX-ray tube having a target area for the electrons may be manufacturedfrom many tungsten alloys which in the known method gave rise todifficulties because their deformabilities at the forging temperaturesused differ too widely from those of the materials of the support.Moreover, the use of the laminated member generally results in animprovement of the adherence to the molybdenum alloy support.

The laminated members may be obtained by various methods, for example byforging together presintered or cast discs, by spraying one of thelayers on to a presintered or cast disc, for example by plasma arcspraying and subsequent sintering of the sprayed layer. Preferably alaminated sintered member is used which is obtained by sinteringtogether layers of powders of the metals or alloys in a mold.

The invention will now be described more fully, by way of example, withreference to the accompanying diagrammatic drawings, in which:

FIG. 1 is a sectional view of a sintered laminated member comprising twolayers,

FIG. 2 is a sectional view of the structure of a set of discs beforeshaping,

FIG. 3 is a sectional view of the structure of this set after shaping,and

FIG. 4 is a sectional view of a finished rotary anode.

In a suitable mold an amount of powdered tungsten or powdered tungstenalloy is scattered in an even layer, after which an amount of thepowdered tungsten alloy for the target area is distributed over thefirst layer. Obviously this order may be reversed. The assembly iscompressed and sintered in a non-oxidizing or reducing atmosphere toform the sintered laminated member 1 shown in FIG. 1. The layer 2consists of the alloy desired for the target area and the layer 3consists of tungsten or a tungsten alloy having a lower yield point andbetter deformability than the first-mentioned tungsten alloy.

The sintered unitary member 1 then is placed on a disc-shaped member 4consisting of a molybdenum alloy with the layer 3 facing the member 4.The assembly is heated to the desired deformation temperature and thensubjected to a high-speed forming impact process.

Subsequently the two pieces 1 and 4 are joined to one another by meansof a flow of high energy content, their diameters being increased andtheir thicknesses being reduced to an at least substantially equalextent (FIG. 3). After the resulting laminated assembly has been takenfrom the press any stresses in its are relieved by annealing. Then theassembly may be bent at points 6 and 7 and may have a hole 5 formed init for receiving a spindle and the edge may be finished, resulting inthe rotary anode shown in FIG. 4.

In order to prevent oxidation of the disc-shaped pieces 1 and 4 duringthe heating and subsequent forming operations they are preferablyenclosed in a thin-walled envelope made of a material which does notmelt during the heat treatment and is inert with respect to thematerials of the discs, which thus are heated and subjected to thehigh-speed forming impact process together with and in the envelope.Such an envelope may consist of tungsten or molybdenum sheet and may beremoved after the forming operation.

The high-speed forming impact process provides very good adherencebetween the two disc-shaped pieces. During the forming process in whichthe thicknesses of the disc-shaped pieces decrease and their diametersincrease, oxide films on their surfaces of contact are broken up andfresh materials from both layers which have not been in contact with theatmosphere before are intimately contacted at a very high pressure andan elevated temperature resulting in an excellent bonding of the piecesto each other.

Sintered members having a density of about 85% to 95% of thetheoretically possible density may suitably be used in the methodaccording to the invention. The porosity of the sintered member can becontrolled in known manner by compressing the material before thesintering and by the manner of sintering. The disc-shaped piece fromwhich the support of the rotary anode is formed preferably consists of amaterial having a density of at least 90% of the theoretically possibledensity.

It has been found that the thickness ratio between the layers of thesintered member is not very critical. If the thickness of the targetlayer which consists of the tungsten alloy is fixed, the thickness ofthe other layer may, for example, be one half of this thickness, beequal to it or be twice this thickness. Any thickness ratio lyingbetween these values may be used. The overall thickness of the sinteredmember may be a few mm.

By the method according to the invention rotary anodes for use in X-raytubes can be made having a target area consisting of alloys of tungstenwith one or more of the metals rhenium, tantalum, osmium, iridium,platinum, ruthenium, rhodium and palladium, and intermediate layerconsisting of tungsten or a tungsten alloy, and a supporting layerconsisting of alloys of molybdenum containing small amounts of one ormore elements chosen from the group consisting of titanium, zirconium,carbon, hafnium and boron.

EXAMPLE I

In a cylindrical mould of a diameter of 50 mm an even layer of powderedtungsten is spread which is then covered by a layer of a powder of atungsten--rhenium alloy containing 4.5% by weight of rhenium. Theassembly then is compressed at a pressure of 2.5 tons/cm² and sinteredin a reducing atmosphere at a temperature of 2200° C.

The resulting disc 1 is placed on a disc 4 of equal diameter consistingof a cast molybdenum alloy containing 0.5% by weight of titanium and0.08% by weight of zirconium. The assembly is heated to a temperature ofabout 1600° C. and then is formed in a pneumo-mechanical machine in asingle blow into a disc having a diameter of 120 mm. The thickness ofthe assembly then is about 8 mm. Any stresses are relieved by annealingat a temperature of 1000° C., after which the assembly is made up into arotary anode by known methods. In this embodiment the thickness of thetungsten-rhenium layer 2 is 0.8 mm, that of the tungsten layer 3 is 1.2mm and that of the support 4 is 4 mm.

Similarly, in manufacturing rotary anodes sintered members were used inwhich the upper layer consisted of a tungsten-rhenium alloy containing4.5% by weight of rhenium and the other layer consisted oftungsten-rhenium alloys chosen from the group consisting of such alloyscomprising 0.5% by weight of rhenium, 1% by weight of rhenium, 1.5% byweight of rhenium and 2% by weight of rhenium respectively. In all casesequal deformation of and good adherence between the sintered member andthe support member were obtained by the forming process.

EXAMPLE II

According to the method described in example I a rotary anode wasmanufactured by joining a sintered member comprising a layer of atungsten--niobium alloy containing 2% by weight of niobium, theremainder being tungsten, and a layer consisting of tungsten by thelatter layer to a disc consisting of the molybdenum alloy mentioned inExample I. The tungsten layer and the tungsten-niobium layer both wereabout 1 mm thick.

EXAMPLE III

According to the method described in example I a rotary anode wasmanufactured by joining a sintered member comprising a layer of atungsten-rhenium-tantalum alloy containing 3% by weight of rhenium and0.4% by weight of tantalum, the remainder being tungsten, and a layerconsisting of tungsten by the tungsten layer to a disc consisting of themolybdenum alloy mentioned in Example I.

What is claimed is:
 1. Method of manufacturing a laminated X-ray anodecomprising:(a) forming a preliminary unitary workpiece by laminating afirst electron target layer of a first tungsten alloy to a second layerof a material being one of tungsten or a second tungsten alloy, saidsecond layer being a material having a lower yield point than that ofsaid first layer; (b) providing a support member of a material being oneof molybdenum or a molybdenum alloy; (c) joining said workpiece to saidsupport member by attaching said second layer to said support member ina high rate impact forming process, said step simultaneously increasingrespective transverse dimensions of said workpiece and said supportmember, and reducing respective thicknesses of said workpiece and saidsupport member; and (d) shaping the resulting assembly into a desiredanode form.
 2. A method according to claim 1, wherein said workpiece isproduced by sintering together in a mold layers of powders of respectiveconstituents of said first and second layers.
 3. A method according toclaim 1, wherein said second layer has a thickness that is from aboutone-half to twice the thickness of said first layer.
 4. A methodaccording to claim 1, wherein said joining step is carried out at a hightemperature.
 5. A method according to claim 4, wherein said hightemperature is 1600° C.
 6. A method according to claim 1, wherein saidjoining step is carried out in a single blow by a pneumo-mechanicaltechnique.
 7. A method according to claim 6, wherein said respectivetransverse dimensions of said workpiece and said support member areincreased by more than two in said joining step.
 8. A method accordingto claim 1, wherein said first layer consists of a firsttungsten-rhenium alloy, said second layer consists of one of tungsten ora second tungsten-rhenium alloy having a lower rhenium content than saidfirst tungsten-rhenium alloy, and said support member consists of amolybdenum alloy having from 0.4% to 0.55% by weight of titanium, from0.06% to 0.12% by weight of zirconium, and the balance of molybdenum. 9.A method of forming an X-ray anode comprising the steps of laminating afirst layer of a tungsten alloy to a second layer of one of tungsten anda second different tungsten alloy, said second layer having a loweryield point than that of said first layer, and joining said laminatedfirst and second layers to a support member of one of molybdenum and amolybdenum alloy by a high energy rate forming process.
 10. A methodaccording to claim 9, wherein said support member is joined to saidsecond layer at a surface opposite to that where said first layer islaminated.
 11. A method according to claim 9, wherein said high energyrate forming process is a single blow pneumo-mechanical technique.
 12. Amethod according to claim 11, wherein said first and second layers andsaid support member have their respective diameters increased by morethan two.
 13. A method according to claim 9, wherein said first layer isan alloy of tungsten and at least one of rhenium, tantalum, osmium,irridium, platinum, ruthenium, rhodium and palladium, and said supportmember is an alloy of molybdenum and at least one of titanium,zirconium, carbon, hafnium and boron.
 14. A method according to claim 9,wherein said first layer is a tungsten-rhenium alloy and said secondlayer is tungsten.
 15. A method according to claim 9, wherein said firstlayer is a tungsten-rhenium alloy containing 4.5% by weight of rhenium,and said second layer is a tungsten-rhenium alloy containing 0.5% byweight to 2% by weight of rhenium.
 16. A method according to claim 9,wherein said first layer is a tungsten-niobiom alloy, and said secondlayer is tungsten.
 17. A method according to claim 9, wherein said firstlayer is a tungsten-rhenium-tantalum alloy, and said second layer istungsten.
 18. A method according to claim 9, wherein said support memberis a cast molybdenum alloy.
 19. A method according to claim 18, whereinsaid cast molybdenum alloy contains 0.5% by weight of titanium, 0.08% byweight of zirconium and the balance of molybdenum.